1. Field of the Invention
This invention relates in general to interferometric techniques for surface characterization. In particular, it relates to a new method for providing illumination at different wavelengths for different interferometric applications.
2. Description of the Related Art
Interferometric profilometry enables the performance of non-contact measurement of a surface with high resolution and high measurement speed. Several widely accepted techniques are available for acquiring light intensity data from correlograms generated by interfering light beams reflected from a test object and a reference surface and for calculating the surface topography of the object using a variety of well known analytical tools.
Most notably, phase-shifting interferometry (PSI) is based on changing the phase difference between two coherent interfering beams using a narrowband light, or at a single wavelength λ, to measure fairly smooth and continuous surfaces. Large-step, rough, or steep-surface measurements, on the other hand, have been traditionally carried out with white-light (or broadband light) vertical-scanning interferometry (VSI). Combinations and variations of PSI and VSI have been introduced in the art over the years to improve data acquisition and the quality of the measurements. See, for example, the so-called enhanced vertical scanning interferometry (EVSI) and the high-definition vertical-scan interferometry (HDVSI) processed described in copending U.S. Ser. No. 11/473,447.
Because different interferometric approaches used for different test surfaces require illumination at different wavelengths for best results, interferometers have incorporated illumination systems that permit switching wavelengths to suit the need. As the availability of reliable light emitting diodes of particular wavelengths has improved, the illumination systems have incorporated various combinations of diodes to provide maximum versatility. Green light is suitable for PSI and white light for VSI; thus, some illumination systems have included both sources (a green LED and a white light LED) and a mechanical switch in front of the instrument's optics to allow switching between the two, as illustrated schematically in FIG. 1. A drawback of this configuration has been the fact that the moving part necessarily introduces vibrations in the system that are highly undesirable for interferometric measurements.
Another approach has been to combine red, green and blue LEDs with two dichroic mirrors operating in reflection, as illustrated schematically in FIG. 2. As is readily understood by one skilled in the art, when all three LEDs are energized, the output of the illumination system is white light. Otherwise, a particular single wavelength may be selected simply by turning off the undesired ones. This configuration eliminates the shutter, thus allowing switching among a variety of wavelengths without mechanical action and attendant vibrations. However, the drawback of the system is the fact that the intensity spectrum of the white light produced by combining red, green and blue, while apparently white, is not sufficiently Gaussian to constitute true white light and is therefore not suitable for interferometry. As those skilled in the art would readily recognize, the correlograms and modulation envelopes produced by interferometric measurements are wide and exhibit so-called “ringing,” as illustrated in FIG. 3. Moreover, this system is relatively expensive due to the large number of components (three LEDs and two dichroic elements).
The number of components in the illumination system could seemingly be reduced significantly simply by using a single white-light source and a movable green filter operating in transmission, as illustrated schematically in FIG. 4. The problem with this solution, however, lies in the fact that it still includes a movable part, the filter, and further that green filters tend to attenuate the incoming white light beyond a useful level of intensity in the output green band. Therefore, this apparently simple solution is not practical and a new illumination system with fewer elements and no mechanical components would be very desirable. The present invention addresses this remaining need.